The invention relates to a method for monitoring seafloor movements.
Such a method is known from U.S. Pat. No. 6,813,564.
This prior art reference discloses a method for monitoring possible subsidence of a surveyed seafloor area, that may be caused by compaction of an underground hydrocarbon reservoir, comprising the following steps:
conducting at least two measurement series (S1 . . . , Si . . . , Sm) each comprising at least one time-indexed depth measurement, with a separation in time between the measurement series on the order of months or years;
conducting each depth measurement on a survey station arranged on a benchmark having fixed vertical and horizontal position relative to the local seafloor;
within each measurement series (Si), conducting at least one stationary time-indexed short-time local reference depth measurement series on at least one short-term local reference station on at least one benchmark, for correcting each depth measurement for short-time (e.g. tidal) depth variations; and
conducting the depth measurements relative to at least one depth measurement at a reference station arranged on the seafloor outside the survey area at least once during each measurement series (Si), the reference station essentially being unaffected by long-term effects taking place due to compaction in the reservoir during the series of measurements (S1 . . . , Si . . . , Sm).
The method known from U.S. Pat. No. 6,813,564 therefore relies on the use of an ‘extra’ stationary time indexed short-time local reference depth measurement series on a local reference station.
In this known approach, existing autonomous long-term sensors carry out pressure measurements asynchronously to each other, but in-sync with a stationary short-time (quasi-continuous) local depth/pressure sensor. This does allow for a correction of the asynchronous long-term sensor measurements for-short term depth/pressure variations. Subsequently, tide-free relative heights can be derived by differencing pressures with respect to the short-term local reference station.
There is a need for an improved method for monitoring seafloor subsidence which is unconstrained with regard to the synchronicity of depth and/or pressure measurements and not require additional stationary short term local reference measurements.